(1) Field of the Invention
The present invention relates to a method and device for measuring a pure resistance of an on-vehicle battery, which is mounted on a vehicle to supply electric power to loads in the vehicle.
(2) Description of the Related Art
In general, a battery can repeatedly be used within a range of its charging capacity by carrying out a charge which compensates a discharge thereof. However, when an unexpected event such as a deep discharge and lack of electrolyte takes place or even when the battery is used for a long period of time, repeated charges cause a rapid deterioration in chargeable capacity that corresponds to the electric power to be supplied to loads by discharge. Consequently, in a state that the chargeable capacity deteriorates as a result of the aging of the battery, an engine of a vehicle might not be started again when a starter motor is started after a stoppage of the engine, even if the discharge exceeding the charge takes place for a short period of time.
In this connection, when a new battery is compared with a battery in which the aging takes place, it is known that the latter has a pure resistance greater than that of the former. This is a reason why the measured pure resistance of a battery is a measure of replacing the battery when the vehicle is periodically inspected. By knowing the pure resistance, the degree of the deterioration of the battery can be defined taking the pure resistance and the polarization resistance component into consideration. When the pure resistance is determined, its value can be used to estimate the open circuit voltage of the battery.
In general, when current is discharged from a battery, it causes a voltage drop in the terminal voltage of the battery. The voltage drop is caused by an internal impedance (combined resistance) of the battery and is divided into a voltage drop due to IR loss (pure resistance, i.e. voltage drop due to ohmic resistance) caused by the structure of the battery and a voltage drop due to the polarization resistance component (activation polarization, concentration polarization) caused by a chemical reaction. As shown in FIG. 11, when a current-voltage characteristic (I-V characteristic) is computed, the voltage drop due to IR loss does not change provided that the battery state remains the same. On the other hand, the voltage drop resulting from the polarization resistance component changes depending upon the magnitude of the current and a period of the discharging time. When various states of the battery are estimated on the basis of the I-V characteristic including such a polarization resistance component, an accurate estimation cannot be obtained. This is a reason why a technique for measuring the pure resistance of the battery is necessary. The pure resistance can be obtained by separating the polarization resistance component out from the voltage drop in the terminal voltage of the battery.
As for an instrument generally used so far for measuring the pure resistance of a battery, the pure resistance is measured when the battery is in its static condition, that is, when neither the voltage rise nor the voltage drop resulting from the polarization and so on takes place in the electrolyte caused by charge or discharge.
As an example, there is a method for computing the pure resistance of a battery in which the battery is subjected to a repeated charge and discharge process by applying an alternating current with a frequency from 1 kHz to 100 kHz and in a condition that no polarization of charge or discharge is stored in the battery the pure resistance is computed from a relationship between voltage and current, which changes in a specific period of time, for example, 1 xcexcsec.
That is, as shown in FIG. 12, after a charge is halted a phenomenon is investigated, in which the voltage recovers rapidly and thereafter gradually recovers. Supposing that said rapid recovery within a period of time xcex94t arises only as a result of the pure resistance R and said gradual recovery arises as a result of the other components (capacitance and inductance components) including the polarization excluding the pure resistance, a change in the voltage and current in a short period of time in each applied cycle of the alternating current with a frequency from 1 kHz to 100 kHz is investigated so as to measure the pure resistance.
However, when an on-vehicle battery is a subject for the above mentioned technique, the static condition of the battery exists in a very limited period of time and such a technique cannot be used when the vehicle is in operation.
Moreover, in the example described above, because the data of the voltage V and current I must be collected in a short period of time, that is, because the sampling during a short periodic period of time together with their analog/digital conversion must be carried out in a specific period of time xcex94t, therefore although it can indeed be used as an instrument which can be used independently, to use it while mounting it on the vehicle is very hard to do. Further, in order to make the computed xcex94V/xcex94I to be accurate each value of xcex94V and xcex94I inevitably must be large, however in the vehicle a case in which such a condition is established is very much limited. Consequently, the technique described above cannot be applied to a case in which the pure resistance of an on-vehicle battery must be measured when the vehicle is in operation.
It is therefore an objective of the present invention to solve the above problem and to provide a method and device for measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be measured even when the vehicle is in operation.
In order to attain the above objective, the present invention provides a method of measuring a pure resistance of an on-vehicle battery for supplying electric power to loads in the vehicle comprising the steps of:
periodically measuring a discharge current and terminal voltage of the battery while a rush current flows into a predetermined constant load of the loads, the rush current first monotonically increasing from zero to a peak value and subsequently monotonically decreasing from the peak value to a steady value;
computing a first approximate expression of a current-voltage characteristic for the increasing discharge current and a second approximate expression of a current-voltage characteristic for the decreasing discharge current, said current-voltage characteristic indicating a correlation between the discharge current and terminal voltage of the battery; and
computing an intermediate value of two values of the terminal voltage change per unit current change at points corresponding to the peak values of the first and second approximate expressions excluding a voltage drop caused by a concentration polarization component when the first and second approximate expressions include the voltage drop, wherein the intermediate value computed is taken as a measurement of the pure resistance of the battery.
With the construction described above, the discharge current and terminal voltage of the battery are measured when an electric power is supplied to a load in a normal operation of a vehicle. The pure resistance of the battery can be computed only by processing the data thus measured.
That is, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be measured even when the vehicle is in operation.
Preferably, the intermediate value is computed by averaging the two values of the terminal voltage change.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be accurately measured when the change in the activation polarization at a point corresponding to the peak value becomes the same.
Preferably, each said value of the terminal voltage change is first multiplied by a corresponding ratio of a monotonic increase period of time and a monotonic decrease period of time of the rush current among a total period of time of the rush current flow, respectively, and subsequently the intermediate value is computed by averaging the two multiplied values of the terminal voltage change.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be accurately measured when the change in the activation polarization at a point corresponding to the peak value does not become the same.
Preferably, if the first and second approximate expressions are quadratic, upon computation of the intermediate value, first and second modified approximate expressions are computed, in which the voltage drop caused by the concentration polarization component is excluded, from the first and second approximate expressions, respectively, and two values of the terminal voltage change per unit current change at points corresponding to the peak values of the first and second modified approximate expressions are computed.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be measured by a simple computation.
Preferably, a difference between the terminal voltage of the battery at a point of the discharge current being zero of the first and second approximate expressions is computed, the difference is characterized as a voltage drop caused by a total concentration polarization component arisen while the rush current increases from zero to a peak value and decreases from the peak value to zero, a voltage drop caused by a concentration polarization component which arises while the rush current increases from zero to the peak value among the total concentration polarization component is computed, and a quadratic expression, a second-order coefficient of which is determined by substituting a value, which is obtained by subtracting said voltage drop from a voltage value corresponding to the peak value, into an expression having the same constant and first-order coefficient as those of the first quadratic approximate expression, is computed as the first modified approximate expression.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the modified approximate expression excluding the concentration polarization component can be accurately obtained and therefore, the pure resistance of the battery can be accurately measured.
Preferably, besides a voltage value excluding a voltage drop caused by the concentration polarization component at the peak value, two voltage values excluding a voltage drop caused by a concentration polarization component between a peak value and zero are computed and a quadratic expression, a coefficient of which is determined by using the three voltage values, is computed as the second modified approximate expression.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be measured without a complicated process.
Preferably, differential values of the first and second modified approximate expression at a peak value are used for computing the intermediate value.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the pure resistance of the battery can be measured by a simple computation.
Preferably, besides a voltage value excluding a voltage drop caused by the concentration polarization component at the peak value, two voltage values excluding a voltage drop caused by a concentration polarization component at zero point and at an intermediate point between a peak value and zero are computed and a quadratic expression, a coefficient of which is determined by using the three voltage values, is computed as the second modified approximate expression.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, which includes a smaller amount of processing needed for computing the approximate expression excluding the concentration polarization component.
Preferably, besides a voltage value excluding a voltage drop caused by the concentration polarization component at the peak value, a voltage value excluding a voltage drop caused by a concentration polarization component at an intermediate point between a peak value and zero is computed, a linear expression determined by connecting two points of said voltage value is computed as the second modified approximate expression, and a gradient of the second modified approximate expression is used for computing the intermediate value.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the process for computing the intermediate value becomes simple.
Preferably, a voltage drop caused by a concentration polarization component arising while the rush current increases from zero to the peak value among the total concentration polarization component is computed by multiplying a difference between the terminal voltage of the battery at a point of the discharge current being zero of the first and second approximate expressions, by a ratio of a current-time product while the rush current increases from zero to a peak value to a current-time product while the rush current first increases from zero to a peak value and subsequently decreases from the peak value to zero.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the first modified approximate expression excluding the concentration polarization component can be accurately obtained. Thereby, the pure resistance of the battery can be accurately measured.
Preferably, difference expression, which is a difference between the first and second approximate expressions, is calculated and thereafter, a voltage drop caused by a concentration polarization component arisen while the rush current changes from zero to the peak value is computed according to the steps of:
calculating a first difference in the terminal voltage of the battery between the first and second approximate expressions at a point of the discharge current being zero;
calculating a second difference between voltage values at a current value being zero and at a current value being twice the peak current value calculated on the basis of said difference expression;
calculating a third difference between voltage values at the peak current value and at twice the peak current value; and
computing the voltage drop by multiplying the first difference by the ratio of the third difference to the second difference.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery by which the method is not affected by a relation between the current-time product and the generation of the concentration polarization. A complicated process becomes unnecessary and therefore, the pure resistance of the battery can be simply and accurately measured.
Preferably, when the rush current, which monotonically increases up to a peak value in a short period of time without an occurrence of concentration polarization, flows into the constant load, the first approximate expression is linear and a gradient of the first approximate expression is used for computing the intermediate value.
With the construction described above, the present invention provides a method of measuring a pure resistance of an on-vehicle battery, by which the process for computing the pure resistance becomes simple and the approximate expression can be simply computed.
In order to attain the above objective, the present invention also provides a device for measuring a pure resistance of an on-vehicle battery for supplying electric power to loads in the vehicle comprising:
measuring means for periodically measuring a discharge current and terminal voltage of the battery while a rush current flows into a predetermined constant load of the loads, the rush current first monotonically increasing from zero to a peak value and subsequently monotonically decreasing from the peak value to a steady value;
first computing means for computing a first approximate expression of a current-voltage characteristic for the increasing discharge current and a second approximate expression of a current-voltage characteristic for the decreasing discharge current, said current-voltage characteristic indicating a correlation between the discharge current and terminal voltage of the battery measured by the measuring means; and
second computing means for computing an intermediate value of two values of the terminal voltage change per unit current change at points corresponding to the peak values of the first and second approximate expressions excluding a voltage drop caused by a concentration polarization component when the first and second approximate expressions include the voltage drop,
wherein the intermediate value computed by the second computing means is measured as the pure resistance of the battery.
With the construction described above, the discharge current and terminal voltage of the battery are measured when an electric power is supplied to a load in a normal operation of a vehicle. The pure resistance of the battery can be computed by processing only the data thus measured.